Successful Solution for High Nitrogen Content Wastewater Treatment from Rendering Plants

Abstract Specific composition of wastewater from rendering plants requires sophisticated approach to treatment. COD content up to 10 g/l and nitrogen up to 2,5 g/l leads to many difficulties during treatment with activated sludge process. Performance of such wastewater treatment plants is generally very unstable: severe problems with filamentous and viscous sludge bulking occur due to macronutrient deficiency, nitrification is often inhibited by high ammonia and nitrite content, no sufficient carbon source for denitrification is available, high temperatures up to 40 °C negatively influence treatment processes. Appropriate control of pH, temperature, biocatalytic nutrient dosage, suitable external carbon source and restriction of inhibitory effects led to successful operation of 4 wastewater treatment plants in the region reaching required effluent standards TIN = 60 mg/l and COD = 200 mg/l.

 Keywords Activated sludge, high nitrogen pollution, nitrification inhibition, rendering plant, sludge bulking

Successful Solution for High Nitrogen Content Wastewater Treatment from Rendering Plants

P. Auterská*, L. Novák**

*ODOUR, s.r.o., Dr. Janskeho 953, CZ 252 28 Cernosice, Czech republic, info@odour.cz

**AQUA-CONTACT Praha, v.o.s.,  Buzulucká 6, CZ-16000 Prague 6, Czech Republic, libor.novak@aqua-contact.cz

Introduction

Over the Czech Republic territory there are 9 rendering plants with total projected capacity of 1700 t/d of  animal material destruction.  In the past years, practically all these plants have encountered severe problems with treatment of wastewater. Some rendering plants have constructed new wastewater treatment plants (WWTP) or realized fundamental intensification steps to existing ones. Nevertheless, expecting improvement of wastewater treatment did not come to such effect to meet required effluent standards. Principal reasons of insufficient wastewater treatment plant performance were no or partial nitrification only, partial denitrification and poor settling characteristics of activated sludge caused by viscous or filamentous bulking.

Technological process for animal material destruction is similar for all these factories. It consists of thermal animal material destruction under minimum temperature of 133 °C and pressure over 3 MPa with duration of 20 minutes as a minimum. During sterilization period the temperature and pressure must be maintained, followed by a period of lower pressure and temperature of about 100 °C. By lowering the pressure, approximately 5 % of water is evaporated. During drying process another 60 % of water is evaporated from the material. Final product contains between 1 to 3 % of water. Condensate from the evaporation is highly polluted wastewater with extremely high content of organic compounds and nitrogen.

In spite of the fact, that some special technologies for treatment of such wastewaters are available at the market at present, in the Czech Republic the treatment has been uniquely based on the low loaded activated sludge technologies constructed within last 10 years. The technological process train usually consists of wastewater pre-treatment with degreasing units followed by homogenisation tanks. Wastewater is pumped to electro flotation units and to biological wastewater treatment with arrangement of denitrification – nitrification reactors (two stage or cascade). Treated wastewater is separated in secondary settling tanks of conventional arrangement or step arrangement in series. Sludge disposal line consists of aerobically stabilized sludge dewatered with belt press or centrifuge.

Plants description

Since 2003, four WWTP with unsatisfactory operational performance were monitored, sampled, technologically analysed and step by step controlled with respect to improve their effluent parameters. The first three plants are situated in the Czech Republic, the 4th in the Slovak Republic. Table 1 shows average wastewater characteristics of 4 plants and required effluent standards based on the Czech and Slovak legislation. Table 2 summarizes basic technological parameters of wastewater treatment plants.

Table 1. Characteristics of influent wastewaters and required effluent standards.

* as TIN for CZ, as TN for SK

Table 2. Technological characteristics of wastewater treatment plants.